In spinal muscular atrophy (SMA) peripheral motor neurones are affected without involvement of the upper motor neurons. Clinical features, with symmetrical weakness and atrophy of the proximal voluntary muscles of legs and arms, are caused by progressive loss of alpha motor neurons in the anterior horns of the spinal cord (1). The majority of patients represent autosomal recessive inheritance. SMA has an incidence of about 1:10.000, and is the most frequent autosomal recessive disorder in humans (1). The disease severity of SMA is highly variable. Four clinical groups are usually defined, depending on age of onset and degree of motor disability.

SMA type I (Werdnig-Hoffmann disease) has onset within the first 6 months of life and is the most severe form. The children will never be able to sit upright, and usually die within the first 2 years of living. SMA type II presents with symptoms after the first 6 months of life, and the children may survive beyond 2 years. They are able to sit but never learn to walk. SMA type III (Kugelberg-Welander disease) patients have onset at 3 or above 3 years of age, are able to sit and walk, and usually do not have a reduced lifespan. SMA type IV has onset at later than 30 years of age with only mild symptoms and a normal life expectancy (1,2).

The degeneration of the anterior horn cells leads to decreases in choline acetyltransferase, an important enzyme in the acetylcholine synthesis pathway. The resulting decreased acetylcholine synthesis leads to an increased sensitivity to muscle relaxants. Succinylcholine is absolutely contraindicated, as it probably will cause a lifethreating hyperkalemia in these patients (3,4), probably because they develop extrasynaptic achetylcholine receptors. Nondepolarizing muscle relaxants should also be avoided whenever possible for patients with SMA, because they may produce a very prolonged muscle relaxation making mechanical ventilation necessary. A literature survey using the words spinal muscular atrophy and general anaesthesia, reveals descriptions of what to be aware of when anaesthesia is necessary, including drugs that are contraindicated, but less information of how to perform anaesthesia as such in these patients. We describe a patient with SMA type III as well as suggestions how to perform general anaesthesia to patients with SMA.

Case report

An 11 year old boy with spinal muscular atrophy type III was admitted to our hospital with acute scrotal pain representing a clinical suggestive diagnosis of torsion of the testis. It was deemed necessary to explore the testis immediately. He had been able to walk without any problems and had until recently participated in football and swimming. He now had to shove his hands on his knees in order to get up from a sitting position and had developed problems doing handwriting. He had a mild asthma and was allergic to pollen, apples, plums, and cats, using cetirizin 10 mg daily. He had not had anaesthesia previously. He weighed 46 kg.

Because of food intake one and half hour before admission, a rapid-sequence induction (RSI) was indicated. Having consulted available textbooks and online literature (PubMed) we planned for this without using muscle relaxants at all, but with a non-depolarizing agent (rocuronium) ready at hand. After standard monitoring (ECG, NIBP, SaO2) was established, 3 minutes of preoxygenation with tidal volume ventilation was commenced. A remifentanil infusion (0,5mg/kg/min) was then started and this was allowed to run for 2 min (to about 1mg/kg remifentanil). At this point the patient was still awake, and was then given an iv bolus dose of 1 mg alfentanil iv (approximately 20micrograms/kg) followed by an iv bolus dose of propofol 4 mg/kg. This produced apnoe and abolished eyelid reflexes almost immediately. At this point a firm cricoid pressure (Sellick’s manouver) was applied and direct intubation using a cuffed endotracheal tube number 6 was then performed using a standard Macintosh blade number 3. The vocal cords were fully visible and relaxed (laryngoscopy Grade 1).

The operation was performed under continuous infusions of propofol and remifentanil (total intravenous anesthesia, TIVA). There was a return to spontaneous respiration almost immediately after this was turned off and the patient was extubated 15 minutes after the operation was finished. Postoperative paintreatment consisted only of paracetamol 1000 mg x 3 and postoperative care went uneventfully.

The patient did not have any torsion of the testis, but torsion of the epidydimis

Discussion

Very little information seems to be immediately available regarding management of anaesthesia to patients with SMA, especially when needing RSI. Watts J.C. described a case using TIVA with alfentanil, propofol and remifentanil, without need of RSI, and using a laryngeal mask to avoid muscle relaxants (5). There is a general controversy as to using regional anaesthesia in patients with neurological disorders. It is not known if regional anaesthesia in patients suffering from SMA might worsen symptoms. Veen A et al reported no anesthesia problems in a case using epidural anaesthesia with bupivacaine 0,25% with epinephrine (dilution 1:200000) and 50 µg fentanyl, combined with a general anaesthesia consisting of propofol for induction, and then sevoflurane maintaining spontaneous breathing using a laryngeal mask (3). Kitson R et al. described a case having used awake fiberoptic intubation due to previously acknowledged difficult intubation, followed by general anesthesia without use of muscle relaxant. Anesthesia was induced using alfentanil and propofol iv boluses, and maintained with isoflurane and nitrous oxide inhalation (6). Awake intubation might always be an option in these patients, also when no intubation difficulties are anticipated. Suxamethonium is contraindicated due to the risk of hyperkalaemia (3-4). Other muscle relaxants, opioids and thiopental may all have prolonged durations of action (5). Whenever used, this may then necessitate use of a ventilator and a prolonged weaning from this in the intensive care unit.

People with SMA type III seem to have a normal lifespan, and therefore they may become in need of surgery during their lifetime TIVA using propofol, remifentanil and/or alfentanil may be an anesthesia of choice to avoid long acting drugs and reduce the need for muscle relaxants. We also considered regional anesthesia, but with SMA it is important not to compromise accessory respiratory muscle function. There is, however, only a relative contraindication to the use of regional anesthesia..

If a muscle relaxant is needed, rocuronium is probablythe agent of choice, also in a rapid sequence induction (6).